This invention relates to a method of manufacturing a magnetic recording medium such as a magnetic recording disc for contact recording.
High recording densities are recently being required of magnetic recording devices. In order to achieve a high recording density, both the linear and track densities must be improved. The linear density is related to the so-called flying height of the head for recording and reproducing such that the flying height must be reduced in order to increase the linear density. The flying height of a head for the commercially available magnetic recording disc drive currently having the largest linear density is about 1500 .ANG.. In order to achieve still higher recording densities, there have been attempts to operate the head in contact, or nearly in contact, with the recording disc during read and write operations. Such a read-write method is referred to as contact recording whereby the head occasionally comes into contact with a part of a magnetic disc for recording and reproducing while the latter rotates at a specified speed. Such a read-write method requires a new type of magnetic discs with improved mechanical durability and improved uniformity in the surface conditions. The flying height of the head is less than about 760 .ANG. (3 microinches) and even if the disc surface is ideally flat, the slider part of the head may come to touch the disc surface due to the vibrations of the actuator or an external shock when the head seeks over the disc. The probability of such an occurrence is particularly high if the average flying height is less than 760 .ANG.. For this reason, the contact recording may be characterized by the range of flying height of the head less than 760 .ANG..
Japanese Patent Publication Tokkai 52-20804 discloses a magnetic recording medium obtained by forming a non-magnetic layer of Ni--P on a substrate of an aluminum alloy, a magnetic layer of Co--Ni--P on this non-magnetic layer, and thereafter a protective layer of polysilicate by coating the magnetic layer with a tetrahydroxy silane solution and then subjecting it to a heat treatment. Japanese Patent Publication Tokkai 61-73227 discloses another magnetic recording medium obtained by forming a non-magnetic layer of Ni--P on an aluminum substrate, a magnetic layer of Co--Ni--P on this non-magnetic layer, a protective layer of polysilicate with hard particulates dispersed therethrough by coating the magnetic layer with a tetrahydroxy silane solution containing these particulates and subjecting it to a heat treatment, and then a lubricant layer comprising perfluoro polyether on this protective layer. More recently, Japanese Patent Application Tokugan 01-138095 disclosed a recording medium which is produced by sequentially forming a magnetic layer and a protective layer on a substrate of a glass or ceramic material and is characterized wherein the surface roughness (R.sub.max) of the substrate is less than 100 .ANG..
One of the most important requirements to be satisfied by a recording medium for contact recording is that there should be no irregular protrusions higher than the flying height of the head. The so-called head crushing by such surface protrusions of a recording medium can be evaluated by a so-called glide test and the requirement mentioned above may be rephrased that the recording medium for contact recording must be able to pass a glide test at a lowest possible value. Since the head usually flies at a height of less than 760 .ANG., recording media for contact recording must be those which can pass a glide test at a lower height than the flying height of the head.
It is evident that textured discs of conventional types cannot be used for the purpose of contact recording because textures, after all, are scratches which are purposefully created for preventing adsorption of the head and include irregular bumps of about 1000 .ANG. in height. Since these bumps are greater than the flying height of the head, head crushing is easily caused by these bumps. Even if the texturing process is omitted, the polished surface of an aluminum alloy substrate plated with Ni--P, for example, generally contains polisher traces such that, even if the average surface roughness (R.sub.a) is as small as 100 .ANG., the maximum surface roughness (R.sub.max) may be 500-1000 .ANG., the surface protrusions and indentations being far from uniform. This is not surprising, however, because it used to be considered important to prevent the adsorption of the head and the surface roughness was intentionally created on the non-magnetic substrate. Thus, there has been no need to polish the plated surface of Ni--P to a degree required for the purpose of contact recording. Conventionally, furthermore, the flying height of the head was generally greater than 1000 .ANG. and there was no serious problem to be encountered even if the surface of the recording medium contained bumps of 1000 .ANG. in height. If use is made of a substrate of an aluminum alloy for contact recording, therefore, care must be taken to reduce the polisher traces such that substrates capable of passing a glide test at 760 .ANG. can be obtained.
Aforementioned Japanese Patent Publication Tokkai 52-20804 discloses a magnetic recording medium having a magnetic layer of Co--Ni--P plated over a mirror-polished Ni--P plated layer but the recording medium according to this reference is not appropriate for contact recording for many reasons. Firstly, since the average roughness (R.sub.a) of its mirror-polished substrate surface is about 400 .ANG. and its maximum roughness (R.sub.max) is therefore thought to be 2000-4000 .ANG., this substrate cannot be considered appropriately polished for the purpose of contact recording. Secondly, it is considered essential to reduce as much as possible the gap between the magnetic layer and the magnetic head if it is desired to increase the recording density of the magnetic recording medium. The protective layer formed over the magnetic layer must also be made thinner than the conventional carbon coating (300-500 .ANG.) and its thickness should be reduced ideally to zero, or at least to less than 200 .ANG.. In reality, however, use is made of a polysilicate film as thick as 1000 .ANG. as the protective layer and hence the separation between the magnetic layer of the recording medium and the magnetic head becomes disadvantageously large. Thirdly, the polysilicate film formed as a protective layer does not serve to reduce friction when it contacts the magnetic head. Fourthly, since the protective layer of an active polysilicate is formed directly on the magnetic layer, this active polysilicate tends to adversely affect the characteristics of the magnetic layer and hence the recording and reproducing characteristics of the recording medium as a whole. Fifthly, it is extremely difficult to form a thin polysilicate film of less than 200 .ANG. in thickness uniformly and directly over a magnetic layer. Sixthly, the polysilicate protective layer is not reliably capable of stopping water which enters from its surface. Hence, the number of defects can increase within an atmosphere with high humidity, contributing to deterioration of the characteristics of the recording medium.
As for the magnetic recording medium according to aforementioned Japanese Patent Publication Tokkai 61-73227 having as protective layer a polysilicate film with hard particulates dispersed therethrough, it has the advantage of improved resistance against abrasion, and a lubricant layer can be more easily maintained thereon. Since use is made of a substrate with average surface roughness (R.sub.a) of 200 .ANG. and maximum surface roughness (R.sub.max) in the range of 1000-2000 .ANG., however, it cannot be considered appropriate for contact recording. Additional disadvantages include, as above, that the protective layer of polysilicate is directly formed on the magnetic layer so as to adversely affect the recording and reproducing characteristics of the latter, that the protective layer is not reliably capable of stopping water which enters from its surface, and that it is extremely difficult to form a thin polysilicate film of less than 200 .ANG. in thickness uniformly and directly over a magnetic layer.
Aforementioned Japanese Patent Application Tokugan 01-138095 discloses a magnetic recording medium having a glass or ceramic substrate with surface roughness (R.sub.max) less than 100 .ANG. but these substrate materials are fragile and hence the cost of fabrication becomes significantly higher than if a substrate of an aluminum alloy is used.